As is known, generally two types of axis control systems are currently commercially available: closed-loop and open-loop.
The first type of control system is generally applied to brushless motors or torque motors or DC motors, which perform a movement by determining in each instant the current position and comparing it with the expected position and changing the torque so as to reach the next point with the least possible error.
The solution described above is characterized by low path repeatability, which becomes even more critical when the number of axes involved and mutually interpolated becomes significant (for example 8 or 10).
In order to improve path repeatability, the only possible solution is to increase the density of the points that constitute such paths: in this case, however, the torques involved and the vibration would increase considerably.
Accordingly, the controller, which must examine a huge number of parameters, risks not being able to control the error of all the axes involved, leading therefore to instability or failure of the system.
The second control system, of the open-loop type, normally uses step motors. The open-loop system does not provide for error minimization, but starts from the assumption that if one aims to reach a particular position at a certain instant, that position is assuredly reached at that precise instant. Clearly, this system is particularly vulnerable to a sudden and unexpected variation of the contrast torque, but apart from this drawback, it allows high repeatability if the path is properly managed.
Currently commercially available closed-loop controllers, in order to be suitable for general use, are characterized by a huge series of parameters that relate to all the dynamic variables involved (speeds, accelerations, masses et cetera) and by the path correction criteria, which are in practice a parameterized dynamic composition of PID (Proportional, Integral, Derivative) control.
Therefore, this solution is feasible only when the intervention times, and therefore the correction times, are long enough to allow the controller to perform all the necessary calculations.
Open-loop controllers, instead, execute exclusively a preset path that always assumes correct positioning. Substantially, there is no correction as a function of error.
Even in open-loop systems, currently there is an ongoing proliferation of parameters aimed at improving the paths constantly, with the result of greatly encumbering the control.